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Boost Control: Boost Control Problems

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Boost Control Problems

09.37

00:00 - Before we look at configuring and setting up electronic boost control, it's important to understand what we can hope to achieve from the system.
00:09 There are a range of fundamental problems with boost control that I often see which can't be fixed by electronic boost control, regardless how well the system is tuned.
00:21 Often I see people trying to use the boost control system as a bandaid to fix a more fundamental problem with the turbo installation and this only results in frustration and wasted time.
00:33 To help avoid these common pitfalls, I'll spend a little time now highlighting the more common problems.
00:41 First of all, it's important to understand that even closed loop boost control will only be as good and as effective as the turbo system will allow.
00:50 This means that if your boost pressure is unstable or inconsistent with no electronic boost control at all, the results you can expect from adding boost control are understandably going to be just as inconsistent.
01:05 Before trying to tune the boost control system I always establish first that the engine is capable of providing stable and consistent boost.
01:15 If it can't, then the mechanical installation will need to be addressed before going any further.
01:21 It's no use, for example, if the boost changes every time you go to full throttle.
01:27 A common area I see problems is with wastegate sizing or wastegate installation.
01:34 The wastegate needs to be able to bypass sufficient exhaust flow to control the turbo charger speed, and hence boost pressure.
01:43 This means that the wastegate needs to be sized relative to the engine and turbo charger size.
01:50 Even on a correctly sized wastegate, the way the wastegate is attached to the exhaust manifold also influences boost control.
01:59 Exhaust gas is lazy and doesn't like changing direction.
02:03 For this reason, it's important to attach a wastegate so that the exhaust flow doesn't need to deviate much to flow through the wastegate.
02:12 If the wastegate is positioned so that the exhaust flow needs to make an abrupt change in direction, or turn back on itself, you'll see boost control problems.
02:24 The most common ways these sort of issues will show up is the boost will rise exponentially as the RPM increases.
02:31 In severe cases, the boost will rise to levels that are too high for the engine and this can obviously be dangerous.
02:39 If you're facing this situation, then there is nothing the boost control system can do to help you.
02:45 The boost control system can only increase the boost pressure, and there is nothing it can do to reduce boosts below the minimum boost we can achieve on the wastegate spring alone.
02:58 I've already covered the aspect of boost control plumbing, but I'll repeat that this is still one of the most common sources of trouble, aside from the actual mechanical turbo installation.
03:10 Any problems with boost plumbing should become apparent when you're doing your initial test with no electronic control.
03:17 You can refer back to the boost control plumbing module for more info on this.
03:23 Another issue we'll commonly see, particularly with engines still running factory-sized turbos, is the boost dropping away at high RPM.
03:33 Stock turbos are invariably sized to offer good boost response at low RPM, and the trade off for achieving this is that back pressure in the exhaust can become excessive at high RPM.
03:46 Basically the turbo charger begins choking the exhaust flow.
03:51 One of the results of this is that the wastegate valve can be forced open by this pressure, and in turn, this prevents the turbo from achieving more boost.
04:01 Using electronic boost control, we'll allow this drop off in boost at high RPM to be corrected to a degree, but it's common once we start raising the boost to find that we end up at the upper limit of the boost control system at high RPM.
04:17 This leads me to discussing boost control range.
04:20 We've already briefly touched on this when we were discussing wastegate spring selection, but it's important to understand that we don't have an unlimited range of control for the boost pressure.
04:31 At some point the exhaust back pressure will become high enough to force the wastegate open and limit the ultimate boost pressure.
04:40 This means that we need to choose a spring pressure for the wastegate that will allow us to achieve our desired range of boost levels.
04:48 Normally, I'd recommend fitting a spring as close as you can get to the minimum boost pressure you want to run.
04:54 Let's say 15 psi, for example.
04:57 Depending on the turbo and engine combination, this might let you increase the boost pressure to perhaps 30 to 35 psi, with the wastegate solenoid at its maximum duty cycle.
05:09 In this situation, there would be little point fitting a seven psi wastegate spring, for example, as it effectively limits the upper range of the boost we can achieve.
05:20 Now I'll discuss over boosting on a gearshift, or quick throttle reapplication.
05:26 Again, if this is an aspect of the basic turbo installation it should show up when we're doing our initial tests with no electronic control implemented.
05:36 There are a couple of common causes for this.
05:40 One would be a sticking or damaged wastegate valve that can't physically move fast enough.
05:46 This would be a fundamental issue with the wastegate that requires either repair or replacement.
05:52 Another problem that can result in over boosting is excessive length or volume in the boost control plumbing.
05:59 There's always going to be a delay in the boost pressure actually making its way through the vacuum plumbing and into the wastegate.
06:07 If you're using excessive length for the boost pressure lines, this can result in a small delay in the wastegate operating.
06:14 The same problem can be created by using silicone vacuum hose that has an inside diameter that's very small.
06:21 This can restrict the flow of air into and out of the wastegate and slow down the valve's response.
06:29 This is more often an issue with external wastegates as the chambers are larger, and hence hold a larger volume of air.
06:37 I recommend keeping the boost control plumbing as short as possible and using appropriately sized vacuum hose to avoid these problems.
06:46 In general, I try to keep the length of boost control plumbing to less than 500 millimetres where possible, and often I'll add appropriately positioned nipples to intercooler piping to help reduce this length.
07:00 When it comes to choosing the correct size of vacuum hose, the fitting on the wastegate is usually a good guide.
07:07 For an internal wastegate, you can typically use vacuum hose with an ID of three to four millimetres.
07:14 For external wastegate, I tend to use a larger hose with an ID of five to six millimetres.
07:21 It's important to ensure that all the hose fittings in the system are a compatible size too.
07:27 When installing and plumbing boost control systems, we want to take care how the vacuum hoses are attached and where they're run.
07:36 A hose failure can easily cause uncontrollable boost and the potential for engine damage, so it pays to think ahead.
07:44 Two of the common things to watch out for are heat, and anything that can physically damage the hose.
07:51 Silicone vacuum hose has a finite range of heat that it can withstand, so if you run it too close to the exhaust manifold or turbo, it's possible to melt through it.
08:01 I recommend using a quality heat sheath on the hose near any heat source, or alternatively, you can construct heat shields around the turbo hot parts.
08:12 Since the engine is not hard mounted in the engine bay there will always be some amount of movement during acceleration and breaking.
08:20 It's important to allow for this movement with the length of your vacuum hose, but at the same time, we want to avoid anything in the engine bay that may cut or damage the lines.
08:30 A sharp edge on a bracket or even the rotating engine fan can be easily overlooked.
08:37 Lastly, it's important to make sure any boost control lines are firmly secured at each point to ensure they can't pop off under boost.
08:47 There are a variety of options to achieve this and my personal preference is to simply use cable ties.
08:53 It's quick and easy to do and leaves you with a reliable connection that won't come off by itself.
09:00 This module can't cover every potential problem you're likely to see in a turbo system.
09:05 However, the ones I've listed are the most common issues I've seen over the last 15 years and understanding them will potentially save you a lot of frustration.
09:16 If you're experiencing boost control issues, it's always easy to blame the boost controller or boost control tuning.
09:23 However, I find it best to go back to basics and eliminate the electronic boost control from the system as a first step.